Method and device for producing three-dimensional objects

ABSTRACT

A method for producing three-dimensional objects from a powder material which is capable of solidification by irradiation with a high-energy beam is disclosed. The method comprises homogeneously pre-heating the powder material by scanning with the high-energy beam along predetermined paths over a pre-heating area so that consecutive paths are separated by a minimum security distance which is adapted to prevent undesirable summation effects in the pre-heating area, and then solidifying the powder material by fusing together the powder material. Apparatus for producing such three-dimensional objects is also disclosed.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a divisional of U.S. patent application Ser.No. 12/309,849, filed on Aug. 11, 2009, which application is a nationalphase entry under 35 U.S.C. §371 of International Application No.PCT/SE2006/000917, filed Jul. 27, 2006, all of which are herebyincorporated herein by reference

FIELD OF THE INVENTION

The present invention relates to a method and apparatus for producingthree-dimensional objects layer by layer using a powdery material whichcan be solidified by irradiating it with a high-energy beam. Inparticular, the present invention relates to a powder pre-heatingprocess using an electron beam.

BACKGROUND OF THE INVENTION

Equipment for producing a three-dimensional object layer by layer usinga powdery material which can be solidified, or fused together, byirradiating it with a high-energy beam of electromagnetic radiation orelectrons are known from e.g. U.S. Pat. Nos. 4,863,538 and 5,647,931 andSwedish Patent No. 524,467. Such equipment includes for instance asupply of powder, means for applying a layer of powder on a verticallyadjustable platform or working area, and means for directing the beamover the working area. The powder sinters or melts and solidifies as thebeam moves over the working area.

When melting or sintering a powder using a high-energy beam, it isimportant to avoid exceeding the vaporization temperature of the powder,since otherwise the powder will merely vaporize instead of forming theintended product. U.S. Patent Publication No. 2005/0186538 discloses amethod focusing on this problem. In this method a laser beam isrepeatedly directed to the same powder target area during themelting/sintering phase so as to stepwise raise the powder temperature.In this manner, too high a powder temperature is avoided.

When using an electron beam instead of a laser beam, the situation is insome ways different. As the electron beam hits the powder, a chargedistribution develops around the electron target area. If the chargedistribution density exceeds a critical limit, an electrical dischargewill occur since the powder particles will repel each other. The resultof such a discharge is that the structure of the powder layer will bedestroyed. Applying the method according to U.S. Patent Publication No.2005/0186538 to a powder melting/sintering device equipped with anelectron beam is likely to give a poor result since no measures aretaken in that method to avoid such discharges.

One solution to the problem of avoiding discharges is to add conductivematerial, such as carbon, to the powder so as to increase the electricalconductivity of the powder. Disadvantages of this solution are, however,that the solidifying process of such a powder mixture may be difficultto control and that the properties of the formed product may be affectedin a negative way. For instance, the mechanical strength may bedecreased.

One object of the present invention is to provide a method and apparatusfor the layered production of three-dimensional objects from a powderymaterial, which method and apparatus allow for a controlled and properfusing together of the powdery material, and which are well suited forboth an electron beam and a laser beam.

SUMMARY OF THE INVENTION

These and other objects have now been realized by the discovery of amethod for producing three-dimensional objects from a powder materialcapable of solidification by irradiation with a high energy beam, themethod comprising homogeneously pre-heating the powder material byscanning a predetermined pre-heating area including the powder materialwith the high-energy beam along a plurality of predetermined pathsdistributed over the predetermined pre-heating area in a manner suchthat consecutive paths of the plurality of predetermined paths areseparated by at least a predetermined minimum security distance adaptedto prevent undesirable summation effects in the predeterminedpre-heating area and solidifying the powder material by fusing togetherthe powder material. Preferably, the homogeneously pre-heating includesrescanning the predetermined pre-heating area with the high-energy beam.In a preferred embodiment, the plurality of predetermined pathscomprises a first plurality of predetermined paths and the rescanningcomprises scanning the predetermined pre-heating area along a secondplurality of predetermined paths distributed over the predeterminedpre-heating area in a manner such that consecutive paths of the secondplurality of predetermined paths are separated by at least aninterspacing distance which is less than the predetermined minimumsecurity distance.

In accordance with one embodiment of the method of the presentinvention, the method includes increasing the power of the high-energybeam during the homogeneously pre-heating of the powder material. In apreferred embodiment, the increasing of the power comprises a stepwiseincrease between consecutive scanning of the plurality of predeterminedpaths.

In accordance with another embodiment of the method of the presentinvention, the high-energy beam comprises an electron beam, and theincreasing of the power comprises increasing the current of the electronbeam.

In accordance with another embodiment of the method of the presentinvention, each of the plurality of predetermined paths are scanned fromone end of the predetermined paths to the other end of the predeterminedpaths.

In accordance with another embodiment of the method of the presentinvention, each of the plurality of predetermined paths comprisesparallel paths. In another embodiment, each of the plurality ofpredetermined paths comprises a straight line.

In accordance with another embodiment of the method of the presentinvention, the powder material covers a predetermined powder area, andthe predetermined pre-heating area is greater than the predeterminedpowder area by a predetermined security margin.

In accordance with the present invention, these and other objects havealso been realized by the invention of apparatus for producingthree-dimensional objects from a powder material capable ofsolidification by irradiation with a high-energy beam, the apparatuscomprising scanning means for homogeneously pre-heating the powdermaterial by scanning a predetermined pre-heating area including thepowder material with the high-energy beam along a plurality ofpredetermined paths distributed over the predetermined pre-heating areain a manner such that consecutive paths of the plurality ofpredetermined paths are separated by at least a predetermined minimumsecurity distance adapted to prevent undesirable summation effects inthe predetermined heating area and fusing means for solidifying thepowder material by fusing together the powder material.

The present invention concerns a method for producing three-dimensionalobjects layer by layer using a powdery material, which can be solidifiedby irradiating it with a high-energy beam. The present invention ischaracterized by a method which comprises a pre-heating step with thegeneral purpose of pre-heating the powdery material in a homogeneousmanner, followed by a solidifying step with the general purpose offusing together the powdery material, wherein the pre-heating stepcomprises the sub-step of scanning a pre-heating powder layer area byscanning the beam along paths distributed over the pre-heating powderlayer area, wherein consecutively scanned paths are separated by, atleast, a minimum security distance, the minimum security distance beingadapted to prevent undesirable summation effects in the pre-heatingpowder layer area from the consecutively scanned paths.

An advantage of the present invention is that the pre-heating stepallows the powder layer to be homogenously heated so as to avoid havingtoo large a temperature gradient in the interface between melted metaland powder in the subsequent solidifying step. By using a securitydistance adapted to prevent summation effects from consecutively scannedpaths, it is possible to prevent the energy deposited to the powderduring scanning of a first path adding to the energy deposited duringscan of a second path scanned directly after the first path. Thus, largetemperature gradients can also be avoided during the pre-heating stage.

When using an electron beam the pre-heating has a further advantageouseffect in that it increases the electrical conductivity of the powder.This, in turn, has the effect that a high beam current can be used inthe subsequent solidifying step. Also, the security distance has anadditional advantage when using an electron beam in that it eliminatesthe risk of forming too large a charge density in the relatively coolpowder during the pre-heating step. Thus, powder discharge is prevented.

The inventive use of the beam for pre-heating the powder has severaladvantages compared to the rather obvious alternative to heat up anentire powder bed using e.g. heating elements. One advantage is that nofurther heating equipment is needed. Another advantage is that only thepart of the powder bed that really needs to be heated, i.e. a fractionof the upper layer of the powder bed, is actually heated. This makes theprocess very efficient.

In a first advantageous embodiment of the method of the presentinvention the pre-heating step further comprises the sub-step ofre-scanning the pre-heating powder layer area. In this manner, thepre-heating area can be gradually and homogeneously heated up.Preferably, the paths followed during a re-scan of the pre-heatingpowder layer area are displaced an interspacing distance in relation tothe paths followed during a previous scan of the pre-heating powderlayer area, wherein the interspacing distance is less than the minimumsecurity distance. In this manner, it is possible to obtain ahomogeneously pre-heated powder layer area in situations where it isnecessary to use a scanning pattern where the paths are physicallyseparated by a distance that is shorter than the minimum securitydistance, i.e. in situations where additional, more closely positionedpaths are needed.

In a second advantageous embodiment of the inventive method of thepresent invention, the power of the beam is increased during thepre-heating step. This has the advantage that the beam power initiallycan be kept at a sufficiently low level to avoid large charge densitiesand/or temperature gradients, but also that the beam power increases soas the temperature of the powder increases as to speed up thepre-heating process as much as possible. In a preferred variant of themethod of the present invention, the power of the beam is increasedstepwisebetween consecutive scans or re-scans of the pre-heating powderlayer area. This makes it relatively easy to control the process andallows the powder pre-heating area to be heated in a uniform manner.

In a third advantageous embodiment of the method of the presentinvention, the beam is an electron beam wherein the beam power isincreased by increasing the beam current.

Preferably, the paths form substantially straight and parallel lines.Such paths simplifies the labor of finding a path pattern and pathscanning order that works in practice, considering summation effects,etc. Using straight and parallel paths also simplifies the control ofthe beam during scanning. To further simplify the process, the paths arepreferably scanned from one end to the other.

In a fourth advantageous embodiment of the method of the presentinvention, the pre-heating powder layer area is larger than, and therebyforms a security margin with respect to, a corresponding part of thepowder layer that is to be fused together in the subsequent solidifyingstep. Such a security margin ensures that the whole product area,including its close surroundings, is properly pre-heated, i.e. such thatthe temperature and electrical conductivity of the powder layer do notchange abruptly at the outer border of parts that are to be fusedtogether. Without such a security margin it is likely that problems willarise due to a too large temperature gradient and/or a too large chargedensity.

The present invention also concerns a device adapted to be operatedaccording to the method of the present invention.

BRIEF DESCRIPTION OF DRAWINGS

In the following detailed description of the present invention,reference is made to the following figures, in which:

FIG. 1 is a side, elevational, schematic view of an example of a knowndevice for producing a three-dimensional product to which device themethod of the present invention can be applied;

FIG. 2 in a top, elevational view of an example of a first preferredembodiment of the method of the present invention;

FIG. 3 a is a top, elevational view of one example of how to apply themethod of the present invention to one product shape;

FIG. 3 b is a top, elevational view of another example of how to applythe method of the present invention to another product shape; and

FIG. 3 c is a top, elevational view of another example of how to applythe method of the present invention to another product shape.

DETAILED DESCRIPTION

FIG. 1 shows an example of a known device 1 for producing athree-dimensional product. The device 1 comprises a verticallyadjustable work table 2 on which a three-dimensional product 3 is to bebuilt up, one or more powder dispensers 4, means 28 arranged todistribute a thin layer of powder on the work table 2 for forming apowder bed 5, a radiation gun 6 in the form of an electron gun fordelivering energy to the powder bed 5 as to fuse together parts of thepowder bed 5, deflection coils 7 for guiding the electron beam emittedby the radiation gun 6 over said work table 2, and a control unit 8arranged to control the various parts of the device 1. In a typical workcycle, the work table 2 is lowered, a new layer of powder is appliedonto the powder bed 5, and the electron beam is scanned over selectedparts of the upper layer 5′ of the powder bed 5. In principal, thiscycle is repeated until the product is finished. An expert in the fieldis familiar with the general function and composition of devices forproducing a three-dimensional product, both with regard to the typeoutlined in FIG. 1 and to devices equipped with a laser gun instead ofan electron gun.

In the case where an electron beam is used, it is necessary to considerthe charge distribution that is created in the powder as the electronshit the powder bed 5. The invention is, at least partly, based on therealization that the charge distribution density depends on thefollowing parameters: beam current, electron velocity (which is given bythe accelerating voltage), beam scanning velocity, powder material andelectrical conductivity of the powder, i.e. mainly the electricalconductivity between the powder grains. The latter is in turn a functionof several parameters, such as temperature, degree of sintering andpowder grain size/size distribution.

Thus, for a given powder, i.e. a powder of a certain material with acertain grain size distribution, and a given accelerating voltage, it ispossible, by varying the beam current (and thus the beam power) and thebeam scanning velocity, to affect the charge distribution.

By varying these parameters in a controlled way, the electricalconductivity of the powder can gradually be increased by increasing thetemperature of the powder. A powder that has a high temperature obtainsa considerably higher conductivity which results in a lower density ofthe charge distribution since the charges quickly can diffuse over alarge region. This effect is enhanced if the powder is allowed to beslightly sintered during the pre-heating process. When the conductivityhas become sufficiently high, the powder can be fused together, i.e.melted or fully sintered, with arbitrary values of the beam current andbeam scanning velocity.

A preferred embodiment of the inventive method, wherein the conductivityis increased without creating discharges, is shown in FIG. 2. Here, thebeam is scanned along paths distributed in a certain pattern over a partof the powder bed 5 that is to be solidified, for the purpose ofpre-heating the powder. The part of the upper layer 5′ of the powder bed5 subjected to pre-heating is denoted the pre-heating powder layer area10, or only pre-heating area 10 (see also FIG. 3). Reference numbersL_(x) and L_(y) denotes the sides of the, in this example rectangular,pre-heating area 10. The beam follows the paths, indicated by straightand parallel lines P1.1, P1.2 etc, from the left to the right, i.e. fromx=0 to x=L_(x). On the left side of the lines/paths, a code of each pathis given. On the right side of the lines/paths, the order in which thepaths are scanned is given. Thus, the first path to be scanned is P1.1,the next path is P2.1, after that path P3.1 and so on. Suchconsecutively scanned paths are physically separated by a securitydistance ΔY that will be further discussed below.

Depending on the particular conditions, such as dimensions ofpre-heating area 10, beam power and beam scanning velocity, it may benecessary to use a scanning pattern where the paths are physicallyseparated by a distance that is shorter than the minimum securitydistance ΔY in order to obtain a homogeneously pre-heated powder layerarea 10. FIG. 2 shows an example of such a case where additional, moreclosely positioned paths are needed. The additional paths are indicatedwith codes P1.2, P1.3 etc. Adjacent paths, such as P1.20 and P2.1 orP3.2 and P3.3, are physically separated by an interspacing distance δY.As can be seen on the right side of the lines/paths in FIG. 2, adjacentpaths are not scanned in a consecutive order in order to still separateconsecutively scanned paths by the security distance ΔY.

In the example shown in FIG. 2, the pre-heating area 10 can be seen asdivided into five sub-areas, P1 to P5, and in each sub-area the numberof paths to be scanned are 20; e.g. P1.1 to P1.20 in the first sub-areaP1. More generally, the paths can be denoted PM.N, wherein M is thenumber of the sub-area and N is the number of a particular path in thesub-area M. In FIG. 2, M goes from 1 to 5 and N goes from 1 to 20, whichleads to a total number of 100 paths to be scanned. The values of M andN may be varied depending on e.g. the size of the pre-heating area 10and on the desired pre-heating temperature and/or desired degree ofpre-sintering of the powder.

As given by the path scanning order in FIG. 2, the pre-heating area 10is scanned several times in the y-direction, in this example 20 times.The first time the pre-heating area 10 is scanned, the scanningprocedure is such that the first path PM.1 in each sub-area P1 to P5 isscanned. When this initial step is finished, the pre-heating area 10 isre-scanned by scanning the second path PM.2 in each sub-area P1 to P5.In the next re-scan, the third path PM.3 in each sub-area P1 to P5 isscanned, and so on. This procedure may be seen as one single scanningpattern, comprising the N:th path of each sub-area P1-P5, which singlescanning pattern is displaced in the y-direction, i.e. downwards in FIG.2, a distance corresponding to the interspacing distance δY once thescan of all paths in the single scanning pattern has been completed. Inother words, the paths of a re-scan are displaced in parallel a distanceδY relative to the paths of the previous scan. The reference N denotesthe order of the scan or re-scan of the pre-heating area 10, wherein Nstarts with 1 (for the first scan) and goes to a maximum value that, inthis example, is 20 (for the last scan). Below, this maximum value of Nis denoted N_(r).

Each scanning or re-scanning of the pre-heating area 10 has the effectof increasing the temperature of the powder bed that in turn has theeffect of increasing the electrical conductivity of the powder. The beamcurrent can therefore be increased after each (re-)scanning procedure.How much the beam current can be increased between the scans depends onhow much the conductivity can be increased in the preceding scan.

It is important that the beam current, the beam scanning velocity andthe paths to be scanned are adapted such that the charge density aroundthe position where the beam hits the powder is prevented from exceedinga critical limit above which discharge will occur.

A general function for describing the charge density that develops inthe powder in an arbitrary scanning procedure will be a rather complexfunction of time and beam position since the charge density generatedalong one scanned path will be affected by the charge density generatedalong another scanned path if these paths are not very well separated inspace and time. Thus, summation effects between different paths must betaken into account.

In a predetermined scanning procedure using straight and parallelscanning paths, similar to those shown in FIGS. 2 and 3, summationeffects are much easier to control. For a single, straight path thecharge density depends the fraction I/V_(s), where I is the beam currentand V_(s) is the beam scanning velocity relative to the powder bed. Ifthis fraction is too high, too much charge will be deposited to thepowder per path length unit. From a production point of view it isdesirable to increase the temperature in an efficient manner to minimizethe time required for pre-heating the powder. Thus, the beam current andthe beam velocity should be as high as possible without exciding thecritical limit in charge density. However, since charges will remainaround a scanned path for some time the summation of charge densitybetween different scans has to be considered. It is important that thebeam does not return to the same position, or to the close vicinity ofthe same position, until a certain minimum time period t₀ has elapsed.

Thus, for a given path length in a pre-heating area 10 the beam scanningvelocity is not solely determined by the fraction I/V_(s) but also bythe time period t₀ that has to elapse before the beam can return to thesame position. As the charge density decreases not only with time butalso with distance from the previously scanned position, the requiredtime period that must be allowed to elapse before a certain position ofthe powder layer can be scanned decreases with increasing distance fromthe previously scanned position. In a first order approximation thisdistance-dependent security time period, t_(p), can be considered to beindependent of beam current and set to:

t _(p) =t ₀ −k _(r) *r,

where t₀ is the time that has to elapse before the beam can return tothe same position as it was at t=0, r is the distance between the beamposition at t=0 and the new position at time t, and k_(r) is aproportional factor. Here, t_(p) is assumed to have values between 0 andt₀, which means that summation effects are considered to be negligiblefor sufficient large values of r.

Consequently, the scanning of the paths of the pre-heating area 10 mustbe arranged such that the paths become sufficiently separated in timeand/or space so as to avoid undesirable charge summation effects in thepre-heating powder layer area 10. This holds both for charge summationeffects as discussed above and for energy summation effects wherein theamounts of energy deposited along two paths add together so as tolocally raise the temperature too much. Consecutively scanned paths,such as P4.2 and P5.2 in FIG. 2, must be physically separated to ahigher extent than other scanned paths since the time period elapsedbetween consecutively scanned paths is shorter (provided that the pathsare of equal length and an equal time period between start of the scanof each individual path).

From a given beam scanning velocity, V_(s), and a given length of thepaths, L_(x), it is possible to convert the required distance-dependentsecurity time period t_(p) to a minimum security distance ΔY, which iseasier to handle in practice than a minimum time period. The requiredlength of this distance ΔY depends on how fast the beam returns to x=0.Thus, ΔY increases with decreasing length of the paths L_(x) and withincreasing beam scanning velocity V_(s). In FIG. 2, consecutivelyscanned path, such as P4.2 and P5.2, are separated by the minimumsecurity distance ΔY.

As described above, a certain time period must be allowed to elapsebefore different paths can be scanned. In order to reduce the total timerequired for pre-heating the powder, it is important that the beam scansparts of the pre-heating area 10 that are not subject to a “period ofrest” set by t_(p).

In the example below the following parameters are used:

L_(x), L_(y)=lengths of the sides of the pre-heating powder layer area10,V_(s)=beam scanning velocity,I₀=initial beam current,ΔI=beam current increase between re-scanning of the pre-heating area 10,N_(r)=number of times the pre-heating area 10 is scanned,ΔY=distance between two consecutively scanned paths; minimum securitydistance, andδY=distance between two adjacent path; interspacing distance.

For a given powder layer area, i.e. where values of L_(x), L_(y) aregiven, it is possible to empirically obtain the values for V_(s), I₀,ΔI, N_(r), ΔY and δY that are required for a proper pre-heating of thepowder area in question.

Table 1 shows an example of proper values of V_(s), I₀, ΔI, N_(r), ΔYand δY for a certain powder layer area (L_(x), L_(y)), a certainaccelerating voltage (60 kV), and a certain, commercially available,powder (gas atomized ELI Ti6Al4V).

TABLE 1 L_(x) 120 mm L_(y) 120 mm V_(s) 10000 mm/s I₀ 1 Ma ΔI 1 mA N_(r)18 ΔY 24 mm δY 1.2 mm

Assuming that the time required for the beam to “jump” between differentpaths is negligible (which normally is a fair assumption since the“jump” velocity of a beam normally is much greater than its scanningvelocity) and assuming that linear relationships are valid, it ispossible to use the parameter values in table 1 for producingrelationships that can be used for arbitrary values of L_(x) and L_(y).To obtain the most accurate empirical values, these values should beproduced using a powder area that is as small as possible, i.e. thevalues of L_(x) and L_(y) should be as small as possible. However,approximate empirical values, that might be sufficiently accurate, canbe obtained in a faster way by starting out with a larger powder area(larger L_(x) and L_(y)). Preferably, the same value of δY is usedindependently of the values of L_(x) and L_(y) because δY also affectsthe surface finish of the completed three-dimensional product. It isalso important that the total energy deposited per area unit is evenlydistributed in order to keep the temperature as even as possible overthe powder area in question irrespective of the values of L_(x) andL_(y).

With reference to table 1, the following relationships and limitingparameters are valid:

t₀=(L_(y)/ΔY).L_(x)/V_(s),k_(r)=(t₀−L_(x)/V_(s))/ΔY,k₁=I₀/V_(s),k₂=ΔI/V_(s), andk₃=(I₀+N_(r).ΔI).N_(r)/(V_(s).δY.2),where t₀ is the minimum time period that must be allowed to pass beforethe beam returns to (the close vicinity of) a previously scanned path(i.e. t₀ is the time period that has to elapse before the beam canreturn from e.g. line PM.N to PM.N+1); k_(r) is the factor used todetermine the time period that must be allowed to pass before the beamreturns to x=0 at a distance ΔY from a previously scanned path; k₁ isproportional to the maximum amount of charge deposited per mm of thepaths during the first scan of the pre-heating area 10; k₂ isproportional to the maximum charge deposition increase per mm for eachre-scan of the pre-heating area 10; and k₃ is proportional to an averageenergy deposition per mm² required for keeping the powder surface at acertain temperature.

Here, t₀ and k_(r) are minimum values, whereas k₁ and k₂ are maximumvalues that should not be exceeded. The factor k₃ is a form of guidelinevalue but can be seen as a maximum value that should not be exceeded forthe purpose of speeding up the process.

Values of these limiting parameters can be obtained by using theempirically obtained values in table 1. After having obtained theselimiting parameters, they can be used to calculate the five unknownparameters V_(s), I₀, ΔI, N_(r) and ΔY for arbitrary values of L_(x) andL_(y), as long as δY is kept at almost the same value. Some care has tobe taken since the fractions L_(y)/ΔY and ΔY/δY have to be integers.Thus, the parameters may be determined in an iterative way where, forinstance, L_(x) is kept fixed whereas L_(y) and δY are allowed to varysomewhat.

The task of obtaining empirical values, such as those given in table 1,for other types of powders on the basis of the information given in thistext, can be considered to be routine work for a man skilled in the art.A general rule is that t₀, and thus t_(p), increase with decreasingconductivity of the powder. Thus, for a powder with a low conductivity,large values for L_(x), L_(y), V_(s), N_(a) and ΔY might be necessary;in conjunction with low values for I₀ and ΔI.

As described above, pre-heating of the powder layer may be performedover a rectangular powder layer area that encloses all parts of thepowder that are to be fused together. This may, however, be aninefficient approach since, depending on the form of the product to beproduced; an unnecessarily large powder area might be heated up.

FIG. 3 schematically shows, in a vertical view, three examples ofdifferent shapes of powder layers that are to be fused together as toform a part of the product 3. FIG. 3 also shows the correspondingpre-heating areas 10 (dashed lines) and some selected paths P (thinsolid lines) to be followed during the pre-heating scan. FIG. 3 a showsa product 3 that, at least in this particular layer, has an oval shapewith a hole in the middle, whereas FIGS. 3 b and 3 c show products 3having a rectangular and a circular shape, respectively.

As can be seen in FIG. 3, the shapes of the pre-heating areas 10 havethe same principal shape as the product, i.e. as the shape of the powderlayer to be fused, but the pre-heating areas 10 are enlarged as toenclose the parts that are to be melted together. The size of eachpre-heating area 10 is adapted such that a certain security margin 12 isformed with respect to the corresponding part 3 of the powder layer thatis to be fused together. The security margin 12 should be sufficient forensuring that the whole product area 3, including its closesurroundings, is properly pre-heated, i.e. such that the temperature andelectrical conductivity of the powder layer do not change abruptly atthe outer border of the parts that are to be fused together. For theTi6Al4V-powder mentioned in connection with table 1, the security margin12 should be at least 6 mm. Generally, the magnitude of the securitymargin 12 should be increased with decreasing heat and/or electricalconductivity of the powder.

As can be seen in FIGS. 3 a and 3 c, the lengths of the paths may vary.In such cases it may be needed to adjust some parameters, such as theminimum security distance ΔY, to take into account that some paths takesshorter time to scan.

If the powder used has a very low electrical conductivity, and/or if thedistance is unusually long between the parts of the powder layer thatare to be fused together, it may be necessary to pre-heat also parts ofthe powder layer that are not to be fused together in order to allow thebeam to “jump” between the parts that are to be fused together.Otherwise, when repeatedly jumping over the same not-to-be-fused area,the charge distribution density in this area may exceed the criticalvalue.

The term “jump” refers to the situation when the beam quickly is movedfrom one position to another, for instance from an end position of apath to a starting position of the next path to be scanned. In someapplications it may be favourable to “jump” instead of switching thebeam on and off.

Once the pre-heating method step has been concluded, a solidifyingmethod step may follow in which the beam energy can be further increasedas to melt or sinter the powder grains together. By executing thepre-heating method step in a controlled and accurate manner, it ispossible to ensure that the subsequent solidifying step will be properlycarried out.

Although most of the advantages of the inventive method can be achievedwhen using an electron beam, the method is beneficial also in laser beamapplications. One example is that the inventive method is capable ofproducing a homogeneously sintered powder layer area. Such sinteredareas will increase the heat conductivity in the powder and thusminimize the possibility of having too large temperature gradients inthe interface between melted metal and powder in the subsequent meltingstep.

The invention is not limited by the embodiments described above but canbe modified in various ways within the scope of the claims. Forinstance, if the ratio L_(x)/V_(s) is large, it is possible to positionthe subsequent path close to the path just scanned. In such a case thesecurity distance ΔY may be set equal to the interspacing distance δY,i.e. the same paths are scanned in each re-scan.

It is further possible to scan the paths in a different order than whatis described in relation to FIG. 2. For instance, the first path in eachsub-group (P1.1, P2.1, etc.) could be scanned several times before thesecond path in each sub-group (P1.2, P2.2, etc.) is scanned. On someoccasions, in particular if the fraction L_(x)/V_(s) is large, it mayalso be possible to scan the same path several times without scanningany other paths in between.

Moreover, the paths do not necessarily have to be straight and parallellines. However, such a path pattern simplifies the labour of finding apath pattern and path scanning order that works in practice, consideringsummation effects etc. Using straight and parallel paths also simplifiesthe control of the beam during scanning.

Although the invention herein has been described with reference toparticular embodiments, it is to be understood that these embodimentsare merely illustrative of the principles and applications of thepresent invention. It is therefore to be understood that numerousmodifications may be made to the illustrative embodiments and that otherarrangements may be devised without departing from the spirit and scopeof the present invention as defined by the appended claims.

1. Apparatus for producing three-dimensional objects from a powdermaterial capable of solidification by irradiation with a high-energybeam, said apparatus comprising scanning means for homogeneouslypre-heating said powder material by scanning a predetermined pre-heatingarea including said powder material with said high-energy beam along aplurality of predetermined paths distributed over said predeterminedpre-heating area in a manner such that consecutive paths of saidplurality of predetermined paths are separated by at least apredetermined minimum security distance adapted to prevent undesirablesummation effects in said predetermined heating area and fusing meansfor solidifying said powder material by fusing together said powdermaterial.
 2. The apparatus of claim 1 wherein said high energy beamcomprises a high energy electron beam.
 3. The apparatus of claim 1wherein said scanning means includes rescanning means for rescanningsaid predetermined preheating area with said high energy beam.
 4. Theapparatus of claim 3 wherein said scanning means is adapted for scanninga first plurality of predetermined paths and for rescanning saidpredetermined preheating area along a second plurality of predeterminedpaths distributed over said predetermined preheating area in a mannersuch that consecutive paths of said second plurality of predeterminedpaths are separated by at least an interspacing distance which is lessthan said predetermined minimum security distance.
 5. The apparatus ofclaim 1 including power means for increasing the power of said highenergy beam during said homogeneously preheating of said powdermaterial.
 6. The apparatus of claim 5 wherein said power means isadapted to increase said power stepwise between consecutive scanning ofsaid plurality of predetermined paths.
 7. The apparatus of claim 1wherein each of said plurality of predetermined paths comprises parallelpaths.
 8. The apparatus of claim 1 wherein each of said plurality ofpredetermined paths comprises a straight line.
 9. Apparatus forproducing three-dimensional objects from a powder material capable ofsolidification by irradiation with a high-energy beam, said apparatuscomprising scanning means for homogeneously pre-heating said powdermaterial by scanning a predetermined pre-heating area including saidpowder material with said high-energy beam along a plurality ofpredetermined paths distributed over said predetermined pre-heating areain a manner such that consecutive paths of said plurality ofpredetermined paths are separated by at least a predetermined minimumsecurity distance and a predetermined time period adapted to preventundesirable charge summation effects in said predetermined heating areaand fusing means for solidifying said powder material by fusing togethersaid powder material.